Glenn Mulcaire, the former private investigator jailed for intercepting voicemails on phones used by aides to Princes William and Harry at the behest of the News of the World, has run up a legal bill of hundreds of thousands of pounds as he battles a string of ongoing phone-hacking lawsuits.

The expensive defence, estimated to be in excess of £500,000, has triggered speculation that the costs are being paid by the publishers of the tabloid newspaper, whose controlling shareholder, Rupert Murdoch, has said he would take "immediate action" against anybody found to be caught hacking again.

Mulcaire's costs are likely to rise quickly as a string of actions from more public figures suing both him and the newspaper are expected to follow in the next few weeks, adding to the pressure on a south Londoner described as unemployed and receiving jobseeker's allowance in a court judgment in February of last year.

Mulcaire's legal team refuses to say who is paying his bills. When Sarah Webb, his lawyer, was asked if it was known whether News International – owners of News Group Newspapers, the publisher of NotW – was paying his fees, she replied: "No, we don't know that." News International declined to comment.

Mulcaire's defence is a critical part of the legal battles surrounding the phone-hacking case. In November he was asked to say specifically who at the newspaper had ordered him to intercept the calls of celebrities such as Sienna Miller and Elle MacPherson, as well as politician Simon Hughes.

Shortly after, Mulcaire's lawyers asked the court of appeal to review a November judgment in a phone-hacking case involving celebrity publicist Nicola Phillips, asking him to reveal exactly who at NotW instructed him to hack in to the voicemails of public figures. The cost of a court of appeal case could reach £300,000, lawyers say.

Following court actions, notes seized by the Metropolitan police from the office of Mulcaire have been gradually made available to public figures trying to bring legal actions against NotW. Tomorrow another part of Mulcaire's material will be made available to representatives of Sky Andrew, the sports agent, who is bringing his own case against the newspaper.

According to high court documents filed in a case brought on behalf of actress Sienna Miller, Mulcaire had a habit of writing the first name of whoever it was that asked him to conduct hacking in the top left-hand corner of his paperwork. Miller's claim states that Mulcaire repeatedly wrote "Ian" when he hacked into the phones of Miller and her associates – where Ian is alleged to be Ian Edmondson, the assistant editor (news) at the tabloid, who was suspended last month following that claim.

Now Phillips wants Mulcaire to confirm the identities of the men mentioned by their first names in Mulcaire's notes relating to her case – but Mulcaire's legal team argues that the former private investigator could be at risk of incriminating himself if he did so. If Mulcaire were compelled to give evidence in court, his testimony is likely to be decisive in the remaining phone hacking actions.

He worked under contract for the News of the World until 2006 – and took careful notes of who at the newspaper commissioned his services. Detailed paperwork from his office was seized by the Met as part of their investigation into Mulcaire and former royal editor Clive Goodman. Both men were jailed in January 2007 for plotting to intercept voicemails belonging to royal aides, with Mulcaire receiving a six-month sentence.